The invention generally relates to the use of fiber optic sensors in a variety of environments. More particularly, the invention relates to the use of such sensors in a way which corrects the modulation instability that is generated when greater amounts of power are launched into the relevant fiber.
A distributed fiber optic sensor enables a measurand (usually temperature) to be characterised along the length of the fiber as a continuous function of distance. Distributed temperature sensors are used in the following industries and processes: oil well production, electrical power cables, industrial processes, pipelines, fire alarms and tunnels.
In optical time domain reflectometry (OTDR) type sensors, a short pulse of light is launched into the sensing fiber. As the pulse propagates along the fiber a small fraction of light is scattered back towards the sending end. The processing electronics measures the characteristics of the backscattered light as a function of time relative to the input pulse. As the propagation time of the light is known, the perturbations of its characteristics may be spatially resolved along the fiber.
In Brillouin OTDR (BOTDR) the light scattered back is as a result of Brilloulin scattering. Both the intensity and frequency shift of the Brillouin scattered light are dependent oil temperature and strain. Therefore by analysing these parameters it is possible to realise a distributed temperature and/or strain sensor.
The performance of OTDR type sensors is critically dependent on the signal-to-noise ratio (S/N) at the receiver which is directly dependent on the power of the launched pulse. The upper limit to the power of the launched pulse is determined by the onset of non-linear effects in the fiber. For BOTDR systems the first nonlinear effect which degrades system performance as the pulse power is increased is that known as modulation instability. Modulation instability is a result of the intense electric field of the optical pulse causing a change in the refractive index of the fiber. This causes a change in the phase of the light, effectively modulating it and results in the generation of sideband signals. The sidebands then interfere at the receiver with the wanted Brillouin signals and degradation of the S/N occurs leading to possible inaccuracies in the measurements.
Other types of fiber optic sensing systems are also susceptible to modulation instability. These include Raman OTDR systems, Brillouin or Raman optical frequency reflectometry systems, fiber Bragg grating systems, interferometric systems, polarimetric systems, intensity systems, and distributed, single-point, or multi-point systems.
The prior would therefore benefit from a fiber optic sensing system in which optical signals of greater power can be launched into the relevant fiber without the onset of modulation instability.
Thus, there exists a continuing need for an arrangement and/or technique that addresses one or more of the problems that are stated above.